The present invention relates to a device for controlling the composition of a solution consisting of a main solute such as alcohol and a solvent such as water and more particularly a solution-composition controlling device suitable for controlling the composition of alcohol in dampening or moistening water used in the offset presses.
As is well known in the art, in the case of offset printing, dampening water must be applied to a plate. As a device for dampening a plate, that is, as a water system or moistening device, the so-called Molten system in which Molten (water-retaining cloth) is used as a dampening roller for dampening the plate has been widely used.
However, recently the alcohol dampening system in which no Molten roller is used for dampening a plate has been increasingly used instead of the Molten system because stable pattern qualities can be maintained easily and the printing operation and maintenance of a printing press can be much facilitated.
FIG. 26 shows one example of alcohol dampening systems of the type described above. Reference numeral 1 designates a moistening or dampening water tank which is generally maintained about 15.degree. C. by means of a thermostat. The dampening water in the tank 1 is fed by a pump 2 to a water fountain 3 of a printing press. Part of a group of rollers 4 of the water dampening or moistening device of the water system is immersed in the water fountain 3. Therefore, the dampening water in the water fountain 3 is delivered to the upper surface of a printing plate 6 clamped around a cylinder 5 through the group of rollers 4.
Excessive dampening water supplied to the water fountain 3 is returned to the dampening water tank 1 through a pipe line 7.
However, in the alcohol dampening system of the type described above, the dampening rollers themselves have not a high degree of water retaining capability like Molten rollers so that in order to apply a sufficient amount of dampening water on the plate, a surface active agent such as alcohol is added into water so that the surface tension can be decreased and the dispersion of water in ink can be facilitated.
At present, of various alcohols used as a surface active agent, especially isopropyl alcohol is used in most cases and the concentration of alcohol in the dampening water is generally 5.about.20%. Variations in the concentration of alcohol greatly influence the amount of an inking device, that is, the amount of water applied to the plate and consequently the quality of a printed pattern.
It therefore follows that it is absolutely essential to maintain the concentration of alcohol in the dampening water at a predetermined level in the alcohol dampening system of the type described above so that the measurement of the concentration of alcohol is required.
Of various methods for measuring the concentration of alcohol, a method for measuring the concentration of alcohol in terms of the specific gravity of the dampening water measured by a picnometer in accordance with the fact that the specific gravity of alcohol is lower than that of water has been mainly used.
In the alcohol dampening system of the type described above, the dampening water is generally circulated by the pump between the water fountain on the printing press and the dampening water tank so that the dampening water contains a large number of fine air bubbles. When such air bubbles adhere to the picnometer, it becomes difficult to correctly measure the specific gravity. Furthermore, the contaminants such as ink in the dampening water which adhere to the picnometer result in an error in measurement. Therefore, the picnometer must be cleaned at a high frequency and the use of the picnometer is cumbersome.
It has been well known in the art for a long time that, in addition to alcohol, the so-called H solution consisting of gum arabic dissolved in the aqueous solution of phosphoric acid is added into the dampening water.
In the case of offset printing, the dampening water is applied to the plate in the manner described above so that the water-retaining protective films are formed on the nonprinting areas of the plate, thereby preventing the ink from being applied to the non-printing areas. In this case, it is not satisfactory to form the protective films only with water so that the H solution is added as described above.
However, when the concentration of the H solution in the dampening water varies, the protective films vary so that the quality of a printed pattern is adversely affected.
Therefore, when the dampening water is used in printing, it is essential to control not only the concentration of alcohol but also the concentration of H solution.
In order to control the concentration of H solution, it is of course necessary to measure the concentration of H solution. Since the H solution is acidic, its concentration can be measured in terms of the pH (the concentration of hydrogen ions) of the dampening water in a simple manner. Therefore so far the pH meters commercially available in the market have been used to control the concentration of H solution.
Recently, instead of the H solution, the so-called alkaline dampening water containing sodium silicate, sodium phosphate or the like has been also used in some cases. In this case, it is also essential to control the concentration. In the case of the measurement of the concentation, the concentration of the alkali compound in the dampening water is measured not in terms of the pH of the dampening water but in general in terms of electric conductivity of the dampening water.
As described above, the concentration of alcohol and the pH of the dampening water used in printing must be controlled. It follows therefore that the concentration and the pH must be measured. To this end, the picnometers, the pH meters, the conductivity measurement instruments and the like have been used as described above.